1. Field of the Invention
The present invention relates to a diagnostic device that measures at least blood flow, oxygen saturation level in the blood, or the absolute concentration of hemoglobin of blood in a living object injected with a liquid (for example, a physiological salt solution) that is safe for living objects.
2. Description of the Related Art
Supply of blood to various organs of living objects is an essential factor for maintaining the life of living objects and for living objects to function properly. Blood supply to the brain is particularly important. Measuring cerebral blood flow is an indispensable tool for patients where there is a danger of the brain falling into a critical condition. In one conventional technique, a radioactive substance, such as radioactive xenon, is injected into a living object as a tracer. Changes in the tracer are measured by a .gamma.-ray sensor attached to the head of a subject under investigation. Blood flow is then calculated. In another technique, the tracer injected is a pigment, such as cardio-green. By monitoring the flow of the pigment according to changes in the amount of light irradiated from an external source that is absorbed by the pigment. The blood flow is then calculated.
U.S. Pat. No. 4,281,645 describes a device for measuring changes in blood oxygen and blood concentration in the brain (not cerebral blood flow itself). This device uses near-infrared (NIR) light as a light source. Living tissue is comparatively transparent to NIR light. The device also uses an extremely sensitive sensor, such as a photomultiplier tube, in the optical detector. Although previously only measurements of thin body portions such as finger tips and ear lobes was possible, this device allows measurements of the head. This reference describes clinical monitoring of the head which allows measurements of changes in concentration of oxyhemoglobin (HbO.sub.2) and deoxyhemoglobin (Hb) in the blood in the head.
There have been known the following problems with the above-described device. Measurements can not be repeatedly performed, because tracers such as pigments and radioactive materials are undesirable to living objects. Also, countries and organizations often prohibit injecting these tracers into human subjects. Although more stable and more precise than another conventional technique using the Doppler effect which measures blood flow from the wavelength shift of an ultrasonic wave, measurement of blood flow using such tracers is difficult in a clinical situation. Although a device (hereinafter referred to as an NIR monitor) which measures changes in blood concentration and blood oxygen in the head using NIR light can measure the relative change in oxyhemoglobin (HbO.sub.2) and deoxyhemoglobin (Hb), it can not obtain the absolute value, and so direct information relating to the important cerebral blood flow can not be obtained.